It is known in the art that a standard neodymium doped YAG (Nd:YAG) laser crystal can be used to build a laser that emits radiation at a wavelength of 1.44 .mu.m with reasonable efficiency. This wavelength is useful in a variety of applications. For example, the high absorption coefficient of water for radiation at a wavelength of 1.44 .mu.m (.sup.- 26 cm.sup.-1) allows efficient coupling of radiation at this wavelength to biological tissue in surgical applications, and the radiation at a wavelength of 1.44 .mu.m is also classified as "eye-safe." Although these same characteristics can be associated with Cr,Tm, Ho:YAG lasers operating near 2.1 .mu.m, the opportunity to base a laser system upon the standard Nd:YAG technology provides several technological advantages. For example, laser efficiency drops significantly with increasing temperature in the Cr,Tm,Ho:YAG materials, thereby limiting the efficiency of such lasers in high average power applications. In particular, it is known that the average power of a pulsed Ho:YAG laser does not increase linearly as repetition rate is increased. This occurs as a result of the small energy difference between the lower level of laser transitions and the ground level in Cr,Tm,Ho:YAG. Such a characteristic does not apply to Nd:YAG and the problems associated with elevated temperature are, therefore, much less severe in Nd:YAO than in Cr,Tm,Ho:YAG.
The strongest emission line in Nd:YAG is at 1.064 .mu.m and efficient lasers at this wavelength have been widely used in a variety of applications. Indeed, in order to obtain reasonably efficient oscillation at 1.44 .mu.m with Nd:YAG it is required: (1) to suppress oscillation at wavelengths other than 1.44 .mu.m which can deplete the upper level of the 1.44 .mu.m transition and (2) to operate the 1.44 .mu.m laser well above threshold. One consequence of this is that it is essential to suppress oscillation at 1.064 .mu.m. The first requirement above arises from the fact that the gain cross section at 1.44 .mu.m is low and, in particular, it is approximately ten times less than that at 1.064 .mu.m in this material. In addition, oscillation at 1.32-1.36 .mu.m must also be suppressed. As a result, feedback of all of these wavelengths into the excited laser crystal must be kept much lower than that at 1.44 .mu.m.
The second requirement above arises from the fact that the low gain cross section at 1.44 .mu.m also leads to a high threshold for laser oscillation at 1.44 .mu.m.
In spite of taking the above requirements into account, a significant fraction of the output power from a multimode 1.44 .mu.m Nd:YAG laser may be radiation at a wavelength of 1.064 .mu.m. In fact, if a 1.44 .mu.m resonator is misaligned, the relative ratio between output at 1.064 .mu.m and 1.44 .mu.m can become very large, for example, the power at 1.44 .mu.m can go to zero while the 1.064 .mu.m output rises significantly. Such a happenstance, i.e., the emission of radiation at a wavelength of 1.064 .mu.m at a time when radiation at a wavelength of 1.44 .mu.m is expected by a user, is undesirable and possibly dangerous in some applications since the interaction between biological tissue and laser radiation differs significantly for radiation at these two wavelengths. However, it would be desirable for the user to be able to choose either wavelength on demand since both are known to be useful. For example, 1.44 .mu.m radiation is useful for incising and ablating biological tissue whereas 1.064 .mu.m radiation is useful for coagulating blood and, therefore, for providing hemostasis.
In light of the above, there is a need for a method of reducing the amount of spurious radiation from a 1.44 .mu.m laser and for a laser system for providing beams of radiation at wavelengths of 1.44 .mu.m and 1.064 .mu.m on demand.